An arrangement of this type has already been disclosed by DE-240 420. In this case the support for centring the tool pillar comprises a cone, which is displaceable along the tool pillar and whose tip is directed downwards, and horizontal jaws which cooperate with the conical surface of the cone and which are disposed such that, when the cone moves downwards, they are pressed with very great force against the circular opening in the valve housing in order thus to bring about the desired centring. In order to ensure centring, however, the machining tool has to cooperate with the opening which is delimited by the valve seat. This obviously gives rise to serious restrictions in terms of performance of the machining tool. A further disadvantage is that the cone and jaws with holders therefor render it substantially more difficult to inspect the interior of the valve when the valve seat is being machined.
A further example from the technical point of view is disclosed by U.S. Pat. No. 742 313 and U.S. Pat. No. 4,715,149. In the case of the arrangement according to the fire of these publications, jaws in the form of levers are used to secure and centre the arrangement, the jaws being provided with teeth and arranged so that they are urged outwards against the wall of the valve opening, the wall being provided with a thread.
In the case of further known devices of the above-mentioned type, the support is arranged so as to be secured to the valve housing by means of a mounting plate, or a yoke as in the case of the latter publication. The plate or yoke is secured in different ways depending on the design of the valve housing. If the housing is constructed with screws, they can naturally be used but this assumes that the plate has been provided with bores located such that they match the location of the screws. This presents problems if the instrument is to be used for different valve sizes, in particular if it is considered that the tool pillar below the instrument has to be located exactly in the centre of the opening. The instrument can even be secured by means of chains and belts but this evidently involves a serious risk of unsatisfactory machining results.
A difficult problem is that the machining tool wears down comparatively rapidly, such that the arrangement has to be disconnected from the valve housing so that the tool can be changed. This in turn means that, when the tool has been changed, the arrangement has to be re-aligned accurately relative to the housing and the part thereof to be machined. It is awkward and time-consuming to bring about this realignment with the necessary degree of accuracy, in particular as the known centring and securing members generally cover the entire opening in the housing, which makes it difficult to see thereinto.